In a conventional pixel, the electron-hole pairs generated in a photodiode are integrated in a capacitor giving either a voltage signal or an electrical charge signal.
The value of this capacitor determines the maximum charge quantity it is able to receive without saturation since the voltage applied to capacitance is always limited in a pixel. A higher value gives greater integration capacitance hence a wider dynamic range of operation. However, a high value of the integration capacitor gives a tower voltage for the same amount of integrated charge, hence lesser sensitivity. In many cases this integration capacitance is simply the capacitance of the junction of the photodiode.
A typical example of this configuration is a CMOS active pixel sensor known as an APS having three transistors as illustrated in FIG. 1. Said pixel comprises:                a P-type semiconductor substrate 1;        a photoelectric conversion region defined by an N-type doped region NPD forming a PN junction with the substrate and accumulating an amount of charges during exposure to light, the PN junction formed by the photoelectric conversion region and the substrate forming the cathode and anode of a photodiode;        an insulation layer in silicon dioxide on the surface of the substrate 1;        a readout circuit to read a variation in voltage induced by charge accumulation, said readout circuit comprising:                    a reset transistor T1 controlled by a reset signal RST on its gate to reset the voltage of the photodiode to pre-charge, said reset transistor T1 having one electrode connected to the photoelectric conversion region NPD and another electrode connected to the power supply VDD;            a readout transistor T2 the gate of which is connected to the photoelectric conversion region NPD whilst one of its electrodes is connected to the power source voltage VDD;            a select transistor T3 controlled by a select signal SEL applicable to its gate, one of its electrodes being common with the other electrode of the readout transistor T2, and the other being connected to a readout bus COL.                        
The photoelectric charge is therefore self-integrated on the junction capacitance of the photodiode after the pre-charge action of the reset transistor T1, and is then read by switching of the select transistor T3. The readout sequence is illustrated in FIG. 2 which gives a typical operating chronogram of the 3-transistor CMOS active pixel illustrated in FIG. 1.
In this chronogram the following are schematically illustrated in arbitrary value as a function of time: variation 21 of the reset signal RST, variation 22 of the select signal SEL, variation 23 of the voltage VPD at the photoelectric conversion site i.e. the photodiode, and variation 24 of the voltage VCOL on the readout line COL.
At time t1, the photodiode is reset or pre-charged, before imaging, at an initial voltage by means of a signal RST activating the reset transistor T1. At time t2, the reset transistor T1 is deactivated and an initial readout allows determination of said initial voltage for subsequent elimination of offset of a voltage follower in the readout circuit. At time t3, the select transistor T3 is deactivated by means of the select signal SEL, to allow the photodiode to operate under illumination during exposure. The accumulation of the photoelectric charge then causes a voltage drop on the photodiode (VPD). This variation in voltage is read by transistor T2 in voltage follower mode. Finally at time t4, final readout occurs at the end of exposure by means of the select signal SEL, to collect the voltage representing illumination. The final output signal is the difference between the final readout and the initial readout.
It can be ascertained that the added capacitance in addition to that of the photodiode allows an increase in the saturation level limit but considerably degrades the sensitivity of the pixel. As a result, the dynamic range is not much improved. It would be more advantageous to reduce the integration capacitance to obtain good sensitivity. The saturation level limit can be controlled by exposure time, lens opening, etc.
The reduction in integration capacitance is limited by the intrinsic structure of the photodiode. It is the photodiode, via its photoelectric conversion region, which collects the photons and it is therefore not possible to reduce its surface area without losing the efficacy of photon collection, and hence without losing sensitivity. U.S. Pat. No. 6,531,725, U.S. Pat. No. 6,051,447 and U.S. Pat. No. 5,903,021 propose solutions intended to reduce the junction capacitance of the photodiode of such a pixel. These solutions use PN junctions of the photodiode of which the N-type photoelectric conversion region is partly depleted by reverse biasing.
However low capacitance of the photodiode junction entails a noise problem related to resetting of the photodiode, i.e. a switching noise KTC. This reset noise KTC perturbs proper reading of the initial voltage and cannot easily be compensated except by providing complex memory systems for example.
To improve sensitivity and to reduce noise, charge transfer CMOS active pixel structures have been proposed. As illustrated in FIG. 3, a transfer transistor TX is added to a three-transistor active pixel between the photodiode and the readout transistor T2. This transfer transistor called “transfer gate”, allows transfer of the photoelectric charge accumulated in the NPD region towards a floating diffusion node FD formed by a PN junction of very small size. This floating diffusion node FD generally has low capacitance value, hence the charge-voltage conversion gain is strongly increased. By means of this charge transfer, the conversion gain is no longer related to the junction capacitance of the photodiode NPD. Said structure known as a four-transistor structure allows an increase in the sensitivity of the pixel and for example allows envisaging applications requiring strong sensitivity such as night-time vision.
A charge transfer four-transistor pixel such as illustrated in FIG. 3 has another advantage which further improves the sensitivity of the pixel. The reset transistor T1 of the photodiode injects a charge reset noise KTC into the photodiode at the time it is cut off. This noise called KTC is proportional to the square root of the KTC product where K is Boltzmann's constant, T is absolute temperature and C is the capacitance value. In a three-transistor pixel such as the one in FIG. 1, it is quite difficult to compensate for this reset noise KTC since the noise is produced at the start of exposure and the image signal is read at the end of exposure. This accounts for the low sensitivity of an image sensor formed of three-transistor pixels.
In a four-transistor pixel the situation is very different. The reading of the image signal is preceded by a reset of the floating diffusion node FD just before charge transfer. A differential operation is used to eliminate this KTC noise.
FIG. 4 gives the chronogram of a four-transistor active pixel. In this chronogram there is schematically illustrated in arbitrary value and as a function of time: the variation 41 of the reset signal RST, the variation 42 of the signal applied to the gate of the transfer transistor TX, the variation 43 of the select signal SEL, the variation 44 of the voltage VFD at the floating diffusion node FD and the variation 45 of the voltage VCOL on the readout line COL.
At time t1, the floating diffusion node FD is reset, before imaging, at an initial voltage using the signal RST activating the reset transistor T1, and the select transistor T3 is switched on by means of the select signal SEL.
At time t2, the reset transistor T1 is deactivated by means of the signal RST and an initial readout allows determination of said initial voltage. At time t3, the transfer transistor TX is switched on to transfer the charges from the photoelectric conversion region NPD towards the floating diffusion node FD. At time t4, the transfer transistor TX is deactivated, whilst a second readout takes place on the readout line. At time t5, the select transistor T3 is deactivated.
The output signal is the difference between the initial readout and the second readout and is formed by the variation in voltage caused by the accumulated photoelectric charge in the photoelectric conversion region NPD which was transferred to the floating diffusion node FD. The reset noise of the floating node FD is therefore naturally offset by the differential readout circuit.
Therefore the influence of the KTC noise induced by the capacitance of the floating diffusion node FD can effectively be eliminated by differential readout, but the noise induced by the junction capacitance of the photodiode is not.
The invention of the “pinned photodiode” (PPD) allowed overcoming this difficulty. Illustrated in FIG. 3, a pinned photodiode is formed of a photoelectric conversion region, typically diffusion of N-type, forming a PN junction together with the substrate 1 and forming the photodiode, sandwiched between the substrate 1 typically of P-type and a doped region 5, also called passivation region, resulting from surface diffusion generally at very shallow depth of a heavy dose of the same type as the substrate, typically P, which insulates the photoelectric conversion region NPD from the surface of the substrate 1. When the photodiode is biased with a sufficiently high voltage, the photoelectric conversion region NPD is fully depleted of mobile charge. The spatial charges of the photoelectric conversion region NPD attract and accumulate the photoelectrons generated by the photons during the exposure time.
At the end of exposure, the transfer transistor TX transfers these photoelectrons accumulated in the photoelectric conversion region NPD to the floating diffusion node FD where they are converted to voltage. If this transfer is total, the photodiode PPD again becomes free of mobile charge. There is no generation of reset noise KTC in this case.
Total transfer of the photoelectrons is of prime importance not only to eliminate reset noise KTC but also to prevent possible image lag. The patent application filed in France under number FR 1251387 presents a structure for example allowing good functioning of the transfer transistor TX using a simple CMOS process.
One advantage of a four-transistor pixel is that the readout portion can easily be shared by several PPD photodiodes. FIG. 5 shows the principle of this sharing. Two PPD photodiodes are arranged either side of a floating diffusion node FD to which each of the PPD photodiodes can be connected by means of a transfer transistor TX1, TX2. U.S. Pat. No. 7,964,929 and U.S. Pat. No. 7,989,749 give more details on this matter. Through this possible sharing of readout amplification, pixels of very small size can currently be produced often used in cameras of mobile phones.
These different configurations of charge transfer have contributed towards improving the sensitivity limit of pixels, whilst reducing the size thereof. However, the dynamic range of operation has not at all been improved and has even regressed at times. This is because the low capacitance value of the floating diffusion node FD limits the accumulation capacity. For example the CMOS OV7955 sensor containing four-transistor pixels, of size 6 μm×6 μm, by OmniVision Technology, gives excellent sensitivity (12V/lux*s) but saturates at only 6000 photoelectrons.
To extend the dynamic range of operation and to delay saturation, approaches using multiple exposures have been used in a certain number of commercial products. These entail capturing a scene with several imaging parameters and then combining these exposures to obtain an image of with a wider dynamic range. These approaches require complex image processing. It is also often difficult in real time to find optimal imaging parameters in a changing and/or complex environment.
Documents EP 1 354 360 A1, WO 2009/027449 A1 and WO 2010/103464 A1 propose a pixel design in which the photodiode operates in photovoltaic mode, like a solar cell, contrary to conventional designs whereby the photodiode operates as a light-controlled current source. FIG. 6 shows the structure of a pixel having a photodiode in photovoltaic mode described in patent EP 1 354 360. It shows similarity to a conventional three-transistor pixel but has two fundamental differences: the photovoltaic conversion region is short-circuited during reset action and the junction of the photodiode is automatically direct-biased by the electron-hole pairs generated by the incident photons. The image signal is obtained by differential readout between photovoltaic voltage and zero voltage during reset.
In this operating mode, the voltage on the terminals of an open circuit photodiode is measured as a signal. This voltage, according to Schockley's taw, is related to light intensity via a logarithmic relationship. This logarithmic relationship compresses signal development and gives a wider dynamic range of operation. Despite its numerous advantages this photovoltaic functioning has a certain number of drawbacks in particular in terms of sensitivity, which the present invention sets out to overcome.